1. Field of the invention
The present invention relates to a method for the heat-pressure fixing of a toner image to a transfer medium such as plain paper or an overhead projector (OHP) film.
2. Related Background Art
A number of methods are hitherto known as electrophotography as disclosed in U.S. Pat. No. 2,297,691, etc. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by various means utilizing a photoconductive material, developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixation with heat, pressure, heat-and-pressure, or solvent vapor. Methods for development using toners or methods of fixing toner images have been hitherto proposed in variety, and methods suited for any respective image-forming processes have been employed.
In recent years, with regard to the electrophotography there is a demand for higher-speed copying and higher image quality.
As methods of producing toners, it is commonly known to use a process comprising melt-kneading a thermoplastic resin, a coloring agent such as a dye or a pigment and additives such as a charge control agent to effect their uniform dispersion, thereafter cooling the melt-kneaded product, pulverizing the cooled product by means of a pulverizer, and classifying the pulverized product by means of a classifier to have the desired particle diameter.
In the toners produced through the step of such pulverization, there are limitations in adding a material with release properties (hereinafter "release material") such as wax. For example, to obtain sufficient dispersibility of the release material, there are limitations such that i) the material is not liquefied at the kneading temperature at which it is kneaded together with the resin, and ii) the release material must be contained in an amount of not more than about 5% by weight. Because of such limitations, there is a difficulty in improving the fixing performance of the toners produced by pulverization.
When the toner with a release material is produced by pulverization, the release material is present not only in the interiors of toner particles but also on the surfaces of the toner particles. When the toner produced by pulverization is applied in a heat-fixing method, an appropriate amount of low-molecular weight material with low melt viscosity must be added to the toner so that a sufficient releasability can be provided between a fixing roller of a fixing device and the image to be fixed. In the toner produced by pulverization, however, the low-molecular weight material is also present on the surfaces of toner particles. This brings about a problem of blocking of the toner. It is difficult for the toner produced by pulverization to satisfy both the release properties and anti-blocking properties. On the other hand, when a high-molecular weight component free from the problem of blocking is added, it is necessary to add the high-molecular weight component in a large quantity to the toner in order to impart satisfactory anti-offset properties to the toner. In this instance, however, a material with a high melting point or material with a high softening point is added to the toner in a large quantity, and hence the energy used for fixing the toner must be increased (in other words, the fixing must be carried out at a higher temperature and also at a lower speed). In the case of color toners, this may result in a lowering of the transparency of color toner images fixed on transparent transfer media such as OHP films. Moreover, as for color toners suited for the heat-fixing method, a low-molecular weight material highly capable of imparting releasability must be contained to improve the anti-offset properties, and at the same time, satisfying the anti-blocking properties and the transparency after fixing.
Fixing devices used in the heat-fixing method have hitherto employed a system in which an image is fixed while a transfer medium having an unfixed toner image on its surface is held between, and transported through, a fixing roller, kept at a given temperature, and a pressure roller having an elastic layer and brought into pressure contact with the fixing roller.
In the fixing devices of this type, the unfixed toner on the transfer medium may adhere to the surface of the fixing roller that heats the toner and fuses it to the transfer medium, and this adhered toner is often transferred to the next transfer medium (i.e., an offset phenomenon). In particular, in a full-color toner image-forming apparatus, toners are fixed at a relatively high temperature so that plural kinds of color toners may be color-mixed in their nearly molten state. Hence the offset phenomenon occurs with greater frequency in full-color toner image fixation than in single-color toner fixation wherein the toner is merely softened and fixed under application of pressure.
As a means for preventing the offset phenomenon, it is common to incorporate a cross-linked resin component into a toner. This method can be effective for imparting anti-offset properties, but may cause the lowering of heat fusion characteristics of the toner. In order to reproduce a half tone in the multi- or full-color fixation process the plural kinds of color toners must be mixedly present on a transfer medium, must be melted in a good state, and it is preferable that the cross-linked resin component not be contained in the toner. For this reason, in a method of fixing color toners using a heat roller, it is common to prevent high-temperature offset by coating the heat roller with a release material such as silicone oil.
From another aspect, it is known to improve the offsetting to a fixing roller by adding to a toner a release material such as polyethylene wax or polypropylene wax (Japanese Patent Publications No. 52-3304 and No. 57-52574). When the polyethylene wax or polypropylene wax has been added to a color toner in an amount necessary for imparting thereto a satisfactory releasability against the fixing roller, it is difficult to achieve a satisfactory transparency for the color toner image fixed onto a transparent transfer medium.
As another method for solving the problem of the offset, U.S. Pat. No. 3,578,797 proposes a method in which a toner image is heated to its melting point by a heating element, the toner image thereby melted is thereafter cooled into a relatively viscous state, where a transfer medium (a toner bearing medium) having the toner image is separated from a heating web in the state that the toner has a weakened adhesion, so that the toner image can be fixed without causing the offset. U.S. Pat. No. 3,578,797 also discloses a system in which the toner image is heated without bringing the toner image and transfer medium into pressure contact with the heating element, so that the transfer medium is not positively heated and hence, the toner can be melted using a smaller amount of energy than in other methods. When, however, the toner image and transfer medium come into contact with the heating element without the pressure contact, the efficiency of thermal conduction is lowered and it takes a relatively long time for the heating and melting of the toner to occur. In particular, in the case of the full-color toner image, it becomes necessary for the individual color toners to be color-mixed with each other in their nearly molten state. Accordingly, the method disclosed in U.S. Pat. No. 3,578,797 must be further improved when full-color toner images are desired.
Japanese Patent Publication No. 51-29825 proposes that a pressure contact means is added to the fixing method of U.S. Pat. No. 3,578,797 so that the efficiency of thermal conduction can be improved and the toner can be heated and melted in a short time and yet in a satisfactory state. This method makes it possible to sufficiently heat and melt the toner because of the pressure contact employed, and is particularly preferable in the color mixing in the formation of color toner images. Since, however, the pressure is applied at the time the toner is heated and melted, the adhesion between the heating element and the toner may become so strong that separation of the toner image transfer medium from the heating element becomes problematic even after the toner cools. In the Japanese Patent Publication No. 51-29825, Teflon (a trade name of fluorine resins such as polytetrafluoroethylene) with a low surface energy is used on the surface of the heating element so that adhesion between the toner and the heating element can be decreased and the separation of the toner image transfer medium from the heating element can be improved.
In order to improve the fixing of color toners and also prevent the offset phenomenon by the use of the fixing device or fixing method as disclosed in U.S. Pat. No. 3,578,797 or Japanese Patent Publication No. 51 -29825, one may contemplate use of a color toner comprising a toner containing a material having release properties. However, as previously stated it is difficult to obtain satisfactory transparency in the color toner image fixed on the transparent transfer medium, when the polyethylene wax or polypropylene wax has been added to the color toner in an amount necessary for imparting thereto a satisfactory releasability against the heating element. Thus, it is sought to provide a toner suited for a heat-fixing method as well as having excellent release properties.